1. Field
This invention relates to broadband microwave frequency converters and, in particular, to converters incorporating subharmonic mixers.
2. Prior Art
Frequency converters at microwave frequencies typically use a configuration similar to that shown in FIG. 6A. In this Figure, a mixer 602, having an RF input port 601, an IF output port 603, and an LO input port 604, is supplied with LO power from a LO source comprising a frequency source 607, a step recovery diode 606, and a YIG filter 605.
In the operation of the frequency converter of FIG. 6, the frequency source drives the step recovery diode which, in turn, produces a signal output having a spectrum containing a number of harmonics of the frequency source.
The output spectrum of a step recovery diode is shown in FIG. 6B. In this Figure, the ordinate 620 represent amplitude while the abscissa 611 represents frequency. The number of evenly spaced harmonic signals, such as harmonic 610, are referred to as comb frequencies.
In the operation of the frequency converter, a particular comb frequency is selected by the YIG filter. By tuning the YIG filter, different comb frequencies are selected to change the RF frequency which provides a desired IF frequency.
The amplitude of the comb frequencies signals decreases rapidly as higher frequency signals are selected. For example, in FIG. 6B, the amplitude of a comb signal 616 at 26 GHz may be 20 dB lower than a comb signal 615 at 13 GHz, a decrease of 20 dB for only a factor of two increase in frequency.
This decrease in LO power with increasing frequency greatly increases the conversion loss of a conventional mixer at higher frequencies, as illustrated in FIG. 6C. In this Figure, the ordinate 612 represents conversion loss, while the abscissa 614 represents local oscillator power. The conversion loss characteristic 613 can be seen to drop from 15 dB at an LO power of -10 dBm to 55 dB at an LO power of -30 dBm. Stated another way, a 20 dB loss in LO power, due to selecting a comb at twice the starting frequency, will result in a 40 dB increase conversion loss.